In a magnetic recording/reproducing apparatus such as HDD (Hard Disk Drive), it is important to enhance recording capability in order to increase an areal recording density. Therefore, recent attention has been directed to assisted recording that temporarily reduces coercive force of a magnetic recording medium during the recording by applying heat or high-frequency magnetic field. A system using high-frequency magnetic field is called “microwave-assisted magnetic recording (MAMR)”.
In the MAMR, a high-frequency magnetic field having a strong microwave band is applied to a region of a nanometer order to locally excite a recording medium, by which a switching field is reduced. With this state, information is recorded. However, since the MAMR utilizes magnetic resonance, a significant effect of reducing the switching field cannot be realized, unless a high-frequency magnetic field with high frequency proportional to anisotropy magnetic field of the recording medium is used. As one of measures to solve this problem, Japanese Unexamined Patent Application Publication No. 2005-025831 describes a high-frequency oscillation device for generating a high-frequency assisted magnetic field, the device having a structure in which a laminated film with a structure similar to a GMR device (giant magnetoresistance effect device) is sandwiched between electrodes. The high-frequency oscillation device can generate a high-frequency oscillating field on a local area by injecting a conductive electron, having spin fluctuation generated in the GMR structure, into a magnetic body through a non-magnet.
“Microwave Assisted Magnetic Recording”, J.-B. Zhu et al., IEEE Trans. Magn., Vol. 44, No. 1, pp. 125-131 (2008) describes a technique of recording information on a magnetic recording medium having large anisotropy magnetic field. In this technique, spin torque from a spin injection pinned layer (hereinafter referred to as pinned layer) in an oscillator adjacent to a main pole of a perpendicular magnetic head is transmitted to an adjacent high-frequency magnetic field generation layer (FGL: Field Generation Layer) through Cu in an intermediate layer, and magnetization in the FGL is rotated at high speed in a plane to generate microwave (high-frequency magnetic field). “Media damping constant and performance characteristics in microwave assisted magnetic recording with circular as filed”, V. Wang et al., Journal of Applied Physics 105, 07B902 (2009) describes a technique of effectively assisting magnetization reversal of a magnetic recording medium by a structure in which an oscillator is arranged between a main pole of a magnetic recording head and a trailing shield posterior to the main pole, and a rotation direction of the high-frequency magnetic field is changed according to the polarity of the recording magnetic field.